1. Field of the Invention
This invention relates generally to lubrication systems and more particularly to replenishing lubricating oil to a rolling element bearing in response to sensing a reduction in elastohydrohynamic film thickness.
2. Description of the Prior Art
In lubrication systems it is often desirable to maintain minimum drag torque and consistant performance over long periods of time. Increases in life requirements for lubricated parts have motivated the search for a lubrication system that would provide a desired amount of lubricating medium over a long period of time. In bearings, it is critical to provide a substantially exact amount of oil between the frictional surfaces since excessive oil has been shown to cause high drag torques due to excessive viscous drag, reduced stability of the rolling element retainer (the cage) and increased emitted vibration. Insufficient oil produces a thinner elastohydrodynamic film which leads to higher friction, increased wear and, consequently, premature bearing failure. Three basic lubrication systems which are well known include the oil film system, the controlled leak system and the grease system.
The oil film system supplies a light film of oil, in a fixed amount, applied to the surfaces of a bearing prior to being placed in operation. This system is simple, has moderately low drag torque but has limited life expectancy due to the inherently limiited useful life of a lubricant. Since only a small amount of oil is used to minimize viscous drag, and since there is a gradual loss of oil due to evaporation, migration and chemical decomposition, the system has an unacceptably short life. If an attempt is made to increase the life by increasing the amount of oil initially, the result will be higher viscous drag torque.
The controlled leak system is an attempt to maintain a constant amount of oil in an operating system, i.e. replenish lost lubricant. A reservoir is placed outside of the bearing. There is a device between the reservoir and the bearing that is designed to provide a slow constant leakage rate of oil from the reservoir into the bearing. This leakage rate is intended to match the amount of oil being lost in the bearing, thus endeavoring to maintain the quantity of lubricant constant throughout the life of the bearing. When operated correctly, this system provides moderately low drag torque and extended life in comparison to the above-described oil film system. Limitations of the controlled leak system include increased complexity and high sensitivity to environmental and manufacturing tolerances which may cause positive or negative extremes in lubricant feed rate resulting in either starving or flooding the bearing.
The grease system is relatively simple, has considerably longer life than the oil film system and maintains a more consistent film thickness than the controlled leak system. Some limitations of the grease system include increased drag torque, greater difficulty of analysis, and adverse effects on cage stability near the end of the useful life of the lubricant. Grease systems, particularly those using channeling type greases, have provided low drag torque and reasonably long life expectancies.
The foregoing illustrates limitations known to exist in present devices. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.